As described comprehensively in U.S. Pat. No. 4,288,327 the deposition of solids onto heat transfer surfaces of steam generating equipment is a major problem. Common contaminants in boiler feed-water that can form deposits are calcium and magnesium salts, carbonate salts, sulfite, phosphate, siliceous matter and iron oxide. Any foreign matter that is introduced into the boiler in insoluble or particulate form will tend to form deposits on the heat transfer surfaces. These formations will decrease the efficiency under which the heat transfer takes place and can lead to over heating, circulation restrictions, damage to the systems, loss of effectiveness, and increased costs due to cleaning, unscheduled outages, and replacement of equipment. In extreme cases, catastrophic tube failure can occur.
Deposit control agents are frequently added to the feedwater of boilers. Their ultimate objective is to inhibit the formation of deposits on the heat transfer surface sand to facilitate the removal of any deposits in the blowdown. This is accomplished via two mechanisms: a solubilization mechanism, where chelants, or chelant-type molecules, form soluble complexes with the deposit-forming species which are removed in the blowdown; and, an adsorption mechanism, where the deposit control agent adsorbs on the surface of the particulate matter and either inhibits the formation of the deposit, or disperses the deposit that is being formed, and makes it more readily removable.
Phosphates, chelants and polymeric dispersants are frequently used in various combinations in boiler treatment programs. The phosphate is used to precipitate hardness or iron species; the chelants have the ability to complex and prevent the deposition of many cations under boiler water conditions. In higher pressure boilers phosphate is also used for pH control, and since it maintains the system at a pH where corrosion in minimized, it also acts as a corrosion inhibitor. Polymers are used to disperse particulate matter, either the precipitate formed with phosphate treatment, or solid or colloidal matter already present. To some extent, polymers can also act as chelants to solubilize cations.
Polymers that have been used in boiler water treatment include naturally occurring products such as lignosulfonic acids and carboxymethylcelluloses. Synthetic anionic polymers are the more preferred materials recently, and include carboxylated polymers, sulphonated polymers, and polyphosphonic acids. Copolymers incorporating combinations of the above functionalities are also used. Examples of effective synthetic polymers are sulfonated styrene, polymaleic acid or anhydride, copolymers of sulfonated styrene and maleic anhydride. Nonionic polymers do not appear to be effective dispersants in boiler water treatment.
In the use of polymeric dispersants, the polymers are fed to maintain a bulk concentration, which is many times higher than the effective amount of polymer needed for adsorption on the surfaces of the particulate matter or the heat transfer surface. That is, the concentration of polymer on the surface is not only determined by the affinity of the polymer for the surface, but also the equilibrium between the adsorbed species and the bulk species. Thus, where a treatment program might utilize 50 to 100 ppm of a polymeric dispersant, only 1 to 10 ppm of active species might be necessary if the polymer could more effectively be brought into contact with the surfaces in question. The excess dispersant can also contribute to the impurities in the boiler and in the steam produced dispersants can degrade under boiler conditions, leading to organic materials which can be present in the steam, affecting its purity.
In many boiler designs, heat fluxes are not uniform throughout the entire unit due to design miscalculations. It is known that deposit weight densities (DWD) (a measure of amount of boiler deposition) increase as heat fluxes increase, approximately as the square of the heat flux. This non-uniformity in heat transfer can lead to "hot spots" in a boiler where the heat flux can be as much as five times the average heat flux. These hot spots are predisposed to failure. It is often the case that even in an effectively treated boiler there will still be many tube failures in these areas of high heat flux.